D-amino acid oxidases (D-AAOs; EC 1.4.3.3) catalyze the oxidative deamination of D-amino acids to keto acids, accompanied by the liberation of ammonia and hydrogen peroxide. A cofactor of this reaction is FAD which is reoxidized by molecular oxygen to form H2O2.

D-AAO activity was first detected in mammalian tissue in 1935 (Krebs, Biochem. J. 29:77–93 (1935)). In the following decades, D-AAOs from a variety of mammalian tissues, fish, birds and reptiles were reported (Konno, et al., DNA Seq. 10(2):85–91 (1999)); Konno, R., Biochim. Biophys. Acta 1395(2):165–170 (1998)); Koibuchi, et al., Histochem. Cell Biol. 104(5):349–55 (1995)); Konno, et al., Zool Mag (Tokyo), 90:368–73 (1981)).
With respect to microbial D-AAOs, studies have been largely limited to enzymes derived from eukaryotes. Microorganisms whose D-AAOs have been studied include yeasts Rhodotorula gracilis (Pilone, et al., Biochim. Biophys. Acta 914:136–142 (1987)), Trigonopsis variabilis (Berg, et al., Anal. Biochem. 71(1):214–22 (1976)) and several Candida species (Gabler, et al., Enzyme Microbial Technol. 27(8):605–611 (2000)), and moulds Neurospora crassa (Sikora, et al., Mol. Gen. Genet. 186(1):33–9 (1982)), Verticillium luteoalbo and various Fusarium species (Gabler, et al., Enzyme Microbial Technol. 27(8):605–611 (2000); Isogai, et al., J. Biochem. (Tokyo) 108(6):1063–9 (1990)). The D-aspartate and D-glutamate oxidases are a special group of enzymes that exclusively oxidize acidic D-amino acids and derivatives thereof (Fukunaga, et al., J. Ferment. Bioengineer. 85(6):579–583 (1998)); Wakayama, et al., J. Ferment. Bioengineer. 5(5):377–379 (1994)).
The most important field of use of D-AAOs is in the preparation of 7-aminocephalosporanic acid (7-ACA), a starting material for the production of semi-synthetic β-lactam antibiotics (Justiz, et al., J. Org. Chem. 62:9099–9106 (1997); Diez, et al, Biotechnol. Bioeng. 55(1):216–226 (1996)). In this two-stage process, the amino group of the D-aminoadipyl side chain of cephalosporin C is oxidized by D-AAO and ketoadipyl-7-aminocephalosporanic acid is formed.
D-AAOs may also be used in the preparation of L-6-hydroxynorleucine, an intermediate for the antihypertensive agent omapatrilat (Hanson, et al., Bioorg. Med. Chem. 7(10):2247–52 (1999); Patel, Biomol. Eng. 17(6):167–82 (2001)) and as signallers in biosensors (Varadi, et al., Biosens. Bioelectron., 14(3):335–40 (1999); Sarkar, et al., The Analyst 124:865–870 (1999)).
Another function of D-AAO is in the preparation of keto acids from D-amino acids. In this, the keto acid, i.e., the reaction product, must be protected from the H2O2 concurrently formed in order to prevent decarboxylation. By co-immobilization of catalase, both the decarboxylation of the keto acid by H2O2 and the oxidation of D-AAO itself are avoided (Buto, et al., Biotechnol. Bioeng. 44:1288–1294 (1994); Fernandez-Lafuente, et al., Enzyme Microbial Technol. 23:28–33 (1998)).
Practically all of the known D-AAOs have a limited spectrum of amino acids that they are capable of converting. A comparative overview of the substrates which can be used for six D-AAOs is provided by Gabler et al. (Enzyme Microbial Technol 27(8):605–611 (2000)). This reference reports that none of these enzymes are able to effectively utilize basic amino acids as substrates.